This invention relates to a low temperature method of converting silica precursor coatings to ceramic silica coatings. The method comprises applying a silica precursor coating to a substrate, exposing the coating to an environment comprising an amine and subjecting the coating to a temperature sufficient to yield the ceramic coating. The methods of the invention are particularly applicable to applying coatings on electronic devices.
Thin film ceramic coatings on electronic devices and circuits have recently been shown to be valuable for their protective and dielectric effect. As protective agents, these thin films can assure reliability and extended service life of the electronics under a variety of environmental conditions such as moisture, heat and abrasion. As dielectric agents, these films can inhibit electrical conduction in many applications such as the interlevel dielectric in multilayer devices.
The art suggests several methods for the application of ceramic coatings. For instance, Haluska et al, in U.S. Pat. Nos. 4,749,631 and 4,756,977, which are incorporated herein by reference, disclose silica based coatings produced by applying solutions of silicon alkoxides or hydrogen silsesquioxane, respectively, to an electronic device and then heating to temperatures of 200.degree.-1000.degree. C. These same references also describe the application of other silicon-containing coatings such as silicon carbide, silicon nitride or silicon carbonitride onto the initial silica layer for added protection or dielectric effect. The ceramic coatings produced thereby have many desirable characteristics such as microhardness, moisture resistance, ion barrier, adhesion, ductility, tensile strength, low electrical conductance and thermal expansion coefficient matching that make these coatings especially valuable.
The art also suggests forming ceramic coatings in anhydrous ammonia environments. For instance, Haluska et al. in U.S. Pat. Nos. 4,847,162 and 4,842,888 teach the formation of nitrided ceramic coatings by heating hydrogen silsesquioxane resin or silicate esters, respectively, to a temperature of between 200.degree. and 1000.degree. C. in the presence of ammonia. These references teach that anhydrous ammonia creates a reactive atmosphere which allows for low temperature (&gt;200.degree. C.) conversion to nitrided silica coatings. Similarly, Glasser et al. in the Journal of Non-Crystalline Solids 64 (1984) pp. 209-221 teach the formation of nitrided ceramic coatings by heating tetraethoxysilane in the presence of anhydrous ammonia.
In addition, the art teaches the incorporation of various amines into alkyl orthosilicate solutions for use as hydrolysis catalysts. For instance, Hitachi in Japanese Patent J62265129 teaches the incorporation of an amine in a silicon alkoxide solution to form a silica sol for use in producing silica glass. Similarly, Kanegafuchi in Japanese Patent J57118592 teaches the incorporation of an amine into an alcoholic solution of a trialkoxysilane to form a stable composition.
Despite the efficacy of the above coatings, ceramification at temperatures less than 400.degree. C. is so slow that commercial applications are impractical. Utilizing temperatures greater than 400.degree. C., on the other hand, can destroy various temperature sensitive devices. Therefore, a need exists for a method of rapidly applying ceramic coatings at low temperatures.
Jada in U.S. Pat. No. 4,636,440 discloses a method of reducing the drying time for a sol-gel coated substrate comprising exposing the coated substrate to aqueous quaternary ammonium hydroxide and/or alkanol amine compounds. This reference, however, is specifically limited to alkanol amines of the formula H.sub.2 NR.sup.1 OH, wherein R.sup.1 represents a C.sub.1 to C.sub.10 alkylene group.
The present inventors have now discovered that by exposing either of two different types of silica precursor coatings to amines, a ceramic silica coating can be obtained on various substrates, including electronic devices, at temperatures as low as room temperature.